This invention relates to an optical phase modulation encoding and decoding system and method and more particularly to a system enabling transmission of information across an optical communications channel.
Fiber optic technology and more specifically an optical channel provides an attractive alternative for transmitting signals. Direct(intensity) modulation is a term that refers to the control of the intensity of an optical beam being emitted from a laser source via the control of the driving current to the laser source. One method of direct (intensity) modulation is dense wavelength division multiplexing (DWDM) where modulation for each channel is baseband, simple on off keying imposed by an interferometric process at 10 Ghz or higher.
The relationship between the driving current and the optical (intensity) amplitude for direct (intensity) modulation is inherently non-linear and limits the (intensity) amplitude of the modulation imposed onto an emitted optical beam. Consequently, the non-linearity of the intensity modulation technique limits the data transmission capacity of this technique and fails to make substantial utilization of the bandwidth of the optical fiber.
Although the use of optical amplifiers with this technique makes optical transmission over very long distances possible, over shorter distances, e.g. through metropolitan areas, this technique uses far less of the potential bandwidth based upon the available signal to noise ratio for an acceptable bit error rate (BER) that is typically available in these circumstances.
It is therefore an object of this invention to provide a system and method of encoding and decoding phase modulated optical beams that reliably operates at data rates substantially in excess of 100 megabytes per second.
It is a further object of this invention to provide a method of modulating an optical signal beam that operates with substantial linearity between the modulating electrical signal and the emitted and modulated optical signal.
It is a further object of this invention to significantly increase the signaling capacity of a fiber optic link above that provided by current technology in situations where a sufficiently high signal to noise ratio so permits.
It is a further object of this invention to provide a reliable method of transmitting microwave signals over a fiber optic link.
This invention results from the realization that an improved method of encoding and decoding an optical signal can be preferably achieved by phase modulation encoding and transmitting a pair of optical signal beams, and by receiving the pair of optical signal beams and by generating multiple optical fringes (signals) in response to the interference of a pair of phase modulation encoded optical signal beams, sampling each optical fringe and generating a set of at least three phase offset signals in response to and in association with each sampled optical fringe, each phase offset signal having a unique and known phase difference relative to each sampled optical fringe, and by deriving a quantitative representation of the phase modulation encoding of the pair optical signals from each set of phase offset signals and by converting the quantitative representation into an electric signal.
In a preferred embodiment, the phase modulation encoding and decoding system includes and phase modulation encoder and optical transmitter, one or more optical paths, an optical receiver, a multiple fringe generator, a phase sampler and a phase processor.
This invention features hyper-dense wavelength multiplexing system comprising a phase modulation encoding system and a phase modulation decoding system and one or more optical paths for propagating a pair of optical signal beams having a phase modulation.
Preferably, the phase modulation encoding system includes a laser diode that emits an optical signal beam, a divider that divides the optical signal beam into at least two identical optical signal beams, a balanced optical wave guide to modulate the pair of identical optical signal beams and an optical transmitter to transmit the modulated pair of optical signal beams (no longer identical) to an optical receiver via one or more optical paths. Optionally, the optical paths include a fiber optic channel for each modulated optical beam. Optionally, the pair of optical signals are modulated by a Mach-Zehnder modulator.
In some embodiments, the optical phase modulation encoding system may further include a local oscillator and exclude a divider, the local oscillator interoperating with the balanced optical waveguide to modulate one of the pair of optical signals having a phase modulation encoding.
In the preferred embodiment, the optical phase modulation decoding system comprises an optical receiver that receives and outputs a pair of optical signals having a phase modulation encoding, a fringe generator responsive to the pair of optical signals output from the optical receiver configures to generate and output a plurality of optical fringes, a phase sampler responsive to the plurality of optical fringes and configured to generate and output a set of at least three phase offset signals that are sampled from and associated with each optical fringe, each phase offset signal of each set of at least three phase offset signals having a unique and known phase difference relative to the phase of the optical fringe associated with the set of at least three phase offset signals, and a phase processor responsive to each set of at least three phase offset signals and deriving therefrom a quantitative representation of the phase modulation encoding of the pair of phase encoded optical beams.
In some embodiments, the optical phase modulation decoding system may further include a local oscillator that generates one of the pair of optical signals having a phase modulation encoding.
In some embodiments, the fringe generator and the phase sampler comprise a three way coupler. In other embodiments, the fringe generator is configured to interfere the pair of optical signals at a small angle and the phase sampler comprises a detector array or a lenticular screen.
In some embodiments, the phase processor derives the phase modulation encoding of the pair of optical signals by employing ratiometric techniques. Preferably, the quantitative representation of the phase encoding of the pair of optical signals is an electronic signal.
Preferably, the system includes a converter for converting the quantitative representation of the phase modulation encoding of the pair of optical signals and a separator that separates the single aggregate electrical signal into a plurality of sub-carrier electrical signals.
In some embodiments, the receiver comprises a telescope and a Wollaston prism and the pair of optical signals are orthogonally polarized before being transmitted through free space to the telescope from the phase modulation encoding system.